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Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of
 switch_stack
To:     Al Viro <viro@zeniv.linux.org.uk>
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Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-arch <linux-arch@vger.kernel.org>,
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        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
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        alpha <linux-alpha@vger.kernel.org>,
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        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>
From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <9db8096e-677f-3852-6ca4-28143a228ec3@gmail.com>
Date:   Mon, 21 Jun 2021 15:36:41 +1200
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Hi Al,


Am 21.06.2021 um 14:27 schrieb Al Viro:
> On Mon, Jun 21, 2021 at 02:01:18PM +1200, Michael Schmitz wrote:
>> Hi Eric,
>>
>> instrumenting get_reg on m68k and using a similar patch to yours to warn
>> when unsaved registers are accessed on the switch stack, I get a hit from
>> getegid and getegid32, just by running a simple ptrace on ls.
>>
>> Going to wack those two moles now ...
>
> Explain, please.  get_reg() is called by tracer; whose state are you checking?

The check is only triggered when syscall tracing (I set a flag on trace 
entry, and clear that on trace exit)... From the WARN_ONCE stack dump, 
it appears that I get the warning from inside the syscall, not 
syscall_trace().

> Because you are *not* accessing the switch stack of the caller of get_reg().
> And tracee should be in something like syscall_trace() or do_notify_resume();
> both have SAVE_SWITCH_STACK done by the glue...

And that's where my problem may be - I stupidly forgot to set the 'all 
registers saved' flag before calling syscall_trace() ...

I'll fix that and try again. Sorry for the noise!

Cheers,

	Michael


>